A new study has found that the seven planets orbiting the nearby ultra-cool dwarf star TRAPPIST-1 are all made mostly of rock, and some could potentially hold more water than Earth. The planets' densities, now known much more precisely than before, suggest that some of them could have up to 5 percent of their mass in the form of water — about 250 times more than Earth's oceans. The hotter planets closest to their parent star are likely to have dense steamy atmospheres and the more distant ones probably have icy surfaces. In terms of size, density and the amount of radiation it receives from its star, the fourth planet out is the most similar to Earth. It seems to be the rockiest planet of the seven, and has the potential to host liquid water

These spectra show the chemical makeup of the atmospheres of four Earth-size planets orbiting within or near the habitable zone of the nearby star TRAPPIST-1. The habitable zone is a region at a distance from the star where liquid water, the key to life as we know it, could exist on the planets’ surfaces. Credit: NASA/ESA/Z. Levy (STScI

The M8V star TRAPPIST-1 hosts seven roughly Earth-sized planets and is a promising target for exoplanet characterization. Kepler/K2 Campaign 12 observations of TRAPPIST-1 in the optical show an apparent rotational modulation with a 3.3 day period, though that rotational signal is not readily detected in the Spitzer light curve at 4.5 μm. If the rotational modulation is due to starspots, persistent dark spots can be excluded from the lack of photometric variability in the Spitzer light curve. We construct a photometric model for rotational modulation due to photospheric bright spots on TRAPPIST-1 which is consistent with both the Kepler and Spitzer light curves. The maximum-likelihood model with three spots has typical spot sizes of Rspot/R⋆≈0.004 at temperature Tspot≳5300±200 K. We also find that large flares are observed more often when the brightest spot is facing the observer, suggesting a correlation between the position of the bright spots and flare events. In addition, these flares may occur preferentially when the spots are increasing in brightness, which suggests that the 3.3 d periodicity may not be a rotational signal, but rather a characteristic timescale of active regions.

There was a paper that appeared on Nature on 19 Mar 2018 that suggested that the planets at TRAPPIST-1 were far less dense than originally measured, with densities (and consequently compositions) that implied they were volatile-rich and migrated inward from beyond the ice line.

I'm not really qualified to have an opinion on which is better analysis, but it's worth noting that the order of these studies is backwards. The higher-density result of Grimm et al is the more recent work.

An Update on the Potential Habitability of TRAPPIST-1. (Franck Marchis)

« …After 2020, if everything goes well with JWST and if the space telescope provides the superb data that we expect, we might have a crude map of the TRAPPIST-1 planets, similar to the rough image of Pluto made with Hubble Space Telescope and later validated by the New Horizons Spacecraft… » ??

The TRAPPIST-1 system provides an exquisite laboratory for understanding exoplanetary atmospheres and interiors. Their mutual gravitational interactions leads to transit timing variations, from which Grimm et al. (2018) recently measured the planetary masses with precisions ranging from 5% to 12%. Using these masses and the <5% radius measurements on each planet, we apply the method described in Suissa et al. (2018) to infer the minimum and maximum CRF (core radius fraction) of each planet. Further, we modify the maximum limit to account for the fact that a light volatile envelope is excluded for planets b through f. Only planet e is found to have a significant probability of having a non-zero minimum CRF, with a 0.7% false-alarm probability it has no core. Our method further allows us to measure the CRF of planet e to be greater than (49 +/- 7)% but less than (72 +/- 2)%, which is compatible with that of the Earth. TRAPPIST-1e therefore possess a large iron core similar to the Earth, in addition to being Earth-sized and located in the temperature zone.

After publication of our initial mass-radius-composition models for the TRAPPIST-1 system in Unterborn et al. (2018), the planet masses were updated in Grimm et al. (2018). We had originally adopted the data set of Wang et al., 2017 who reported different densities than the updated values. The differences in observed density change the inferred volatile content of the planets. Grimm et al. (2018) report TRAPPIST-1 b, d, f, g, and h as being consistent with <5 wt% water and TRAPPIST-1 c and e has having largely rocky interiors. Here, we present updated results recalculating water fractions and potential alternative compositions using the Grimm et al., 2018 masses. Overall, we can only reproduce the results of Grimm et al., 2018 of planets b, d and g having small water contents if the cores of these planets are small (<23 wt%). We show that, if the cores for these planets are roughly Earth-sized (33 wt%), significant water fractions up to 40 wt% are possible. We show planets c, e, f, and h can have volatile envelopes between 0-35 wt% that are also consistent with being totally oxidized and lacking an Fe-core entirely. We note here that a pure MgSiO3 planet (Fe/Mg = 0) is not the true lowest density end-member mass-radius curve for determining the probability of a planet containing volatiles. All planets that are rocky likely contain some Fe, either within the core or oxidized in the mantle. We argue the true low density end-member for oxidizing systems is instead a planet with the lowest reasonable Fe/Mg and completely core-less. Using this logic, we assert that planets b, d and g likely must have significant volatile layers because the end-member planet models produce masses too high even when uncertainties in both mass and radius are taken into account.

The TRAPPIST-1 planetary system represents an exceptional opportunity for the atmospheric characterization of temperate terrestrial exoplanets with the upcoming James Webb Space Telescope (JWST). Assessing the potential impact of stellar contamination on the planets' transit transmission spectra is an essential precursor step to this characterization. Planetary transits themselves can be used to scan the stellar photosphere and to constrain its heterogeneity through transit depth variations in time and wavelength. In this context, we present our analysis of 169 transits observed in the optical from space with K2 and from the ground with the SPECULOOS and Liverpool telescopes. Combining our measured transit depths with literature results gathered in the mid/near-IR with Spitzer/IRAC and HST/WFC3, we construct the broadband transmission spectra of the TRAPPIST-1 planets over the 0.6-4.5 μm spectral range. While planets b, d, and f spectra show some structures at the 200-300ppm level, the four others are globally flat. Even if we cannot discard their instrumental origins, two scenarios seem to be favored by the data: a stellar photosphere dominated by a few high-latitude giant (cold) spots, or, alternatively, by a few small and hot (3500-4000K) faculae. In both cases, the stellar contamination of the transit transmission spectra is expected to be less dramatic than predicted in recent papers. Nevertheless, based on our results, stellar contamination can still be of comparable or greater order than planetary atmospheric signals at certain wavelengths. Understanding and correcting the effects of stellar heterogeneity therefore appears essential to prepare the exploration of TRAPPIST-1's with JWST.